In 1984, a series of novel silicoaluminophosphate SAPO molecular sieves were developed in the Union Carbide Corporation (UCC) (U.S. Pat. No. 4,440,871). SAPO molecular sieves are crystallized silicoaluminophosphates with three dimensional frameworks formed by PO2+, AlO2−, and SiO2 tetrahedrons. Among this kind of molecular sieves, SAPO-34 with chabazite-type framework contains 8-member ring pore, wherein the orifice size is 0.38 nm×0.38 nm. SAPO-34 has attracted attention because it has shown the excellent catalytic performance in methanol to olefins (MTO) process, due to its proper acidity and pore structure.
SAPO-34 molecular sieve is generally produced by a hydrothermal synthesis process which uses water as the solvent and is conducted in a sealed autoclave. The components for the synthesis comprise an aluminum source, a silicon source, a phosphorus source, a template agent, and deionized water. The silicon source may be chosen from silica sol, active silica, and orthosilicate esters. The aluminum source may be chosen from active alumina, pseudo boehmite, or alkoxy aluminum. Preferable silicon source and aluminum source are silica sol and pseudo boehmite. Phosphorus source is generally 85% phosphoric acid. The template agent commonly used comprises tetraethyl ammonium hydroxide (TEAOH), morpholine (MOR), piperidine, isopropylamine (i-PrNH2), triethylamine (TEA), diethylamine (DEA), dipropylamine, and the like, or a mixture thereof. In the traditional hydrothermal synthesis of SAPO-34, the molar amount of the organic amine template agent used is significantly less than the molar amount of water. Water is used as the continuous phase and the main solvent, and the molar ratio of water to organic amine template agent is generally larger than 10. In our research on hydrothermal synthesis process of SAPO-34 using diethylamine as the template agent, we found that with the amount increase of the template agent gradually, both of the product yield and crystallinity decrease to some degrees, seeing Table 1 in Microporous and Mesoporous Materials, 2008, 114(1-3): 4163.
Concerning the synthesized SAPO molecular sieves, several researchers have reported there is a Si enrichment phenomenon on the crystal surface. The reason is that the initial reaction mixtures to produce SAPO molecular sieves are acid or nearly neutral, and with the proceeding of crystallization, the pH values of the initial reaction mixtures rise gradually because the consumption of phosphoric acid which enters into molecular sieves. In the beginning of crystallization, the silicon source exists generally in the form of polymers. Because of the low isoelectric point, the silicon source decomposes by degrees with the rise of pH value, making the proportion of silicon entering in the framework of SAPO molecular sieves increase, leading to the Si enrichment phenomenon on the crystal surface. For instance, in our research on hydrothermal synthesis process of SAPO-34 using diethylamine as the template agent, we found that a non-uniform distribution of Si in the crystal shows a gradual increase of the Si content from the core to the surface, and the ratio of the Si content (molar ratio Si/(Si+Al+P)) on the surface to the Si content in the crystal bulk is 1.41 (Microporous and Mesoporous Materials, 2008, 114(1-3): 4163). Akolekar et al found that in the SAPO-44 crystal, the molar ratio of the Si content (molar ratio Si/(Si+Al+P)) on the surface to the Si content in the crystal bulk is from 6 to 10 (Colloids and Surfaces A: Physicochemical and Engineering Aspects, 146 (1999) 375-386). In general, SAPO molecular sieves show the character of an obvious Si enrichment phenomenon on the crystal surface, but it has been noticed that, even for the same kind of SAPO molecular sieve, there are obvious difference between the elementary composition on the crystal surface and inside the crystal, which changes with the synthesis conditions and the template agents.
Usually, with the increase of Si content in SAPO molecular sieves, the Si coordination structures change from Si(4Al) to Si(nAl) (n=0 to 4) (in different kind of SAPO molecular sieves, the allowable maximum of single Si distribution in the frameworks are different, seeing J. Phys. Chem., 1994, 98, 9614). The Si coordination structures have significant effect on the acid concentration and the acid intensity, and the acid intensity is enhanced in the order of Si(1Al)>Si(2Al)>Si(3Al)>Si(4Al). In the other hand, the amount of acid center produced by each Si atom decrease with the appearance of Si islands in the framework of SAPO molecular sieves (Si(4Al) is 1, and the others are less 1), leading to the decrease of the acid concentration. It is supposed that using the SAPO molecular sieves as the acid catalyst, the catalytic performance must be effected by the distribution of Si in the framework since the non-uniform distribution of Si in crystal bring the non-uniform distribution of acidity. The enrichment of Si on the surface of crystal indicates that the Si coordination structures on the surface of crystal are more complex than inside the crystal. Weckhuysen et al have reported that in the process of methanol to olefin (MTO), reaction firstly occurs near the surface of crystal, and with the reaction going on, the large coke species form and block the pores progressively, making the diffusion of the products inside the crystal more difficult (Chemistry—A European Journal, 2008, 14, 11320-11327; J. Catal., 2009, 264, 77-87). It indicates that the acid environment on the surface of molecular sieve crystal is very important to the catalytic performance, and it is significant to seek a control method of the degree of Si enrichment on the molecular sieve surfaces.
The elementary analysis of molecular sieve surfaces generally is detected using the XPS method, and the elementary distribution form the core to shell is detected using the EDX method of SEM by line scan after cutting the crystal.
The hydrothermal synthesis of AlPO-21 molecular sieve was reported in European patent 0043562 using N,N,N′,N′-tetramethyl ethylenediamine as the template agent. The synthesis of aluminum phosphate molecular sieve SCS-24 was reported in European patent 0538958 using N,N,N′,N′-tetramethyl ethylenediamine as the template agent. The synthesis of AlPO-21 molecular sieve was reported in U.S. Pat. No. 4,898,660 using N,N,N′,N′-tetramethyl-1,3-diaminopropane and N,N,N′,N′-tetramethyl ethylenediamine as the template agents. The synthesis of SAPO-56 was reported in U.S. Pat. No. 5,370,851 using N,N,N′,N′-tetramethyl-1,6-hexanediamine as the template agent. Wilson et al reported the synthesis of AlPO-17, SAPO-17, and SAPO-56 using N,N,N′,N′-tetramethyl-1,6-hexanediamine as the template agent (Mico. Meso. Mater. 1999, 28(1), 117-126). M. Goepper from France reported the synthesis of AlPO-34 in his doctoral dissertation (Universite Haute Alsace, Mulhouse, France, 1990), using N,N,N′,N′-tetramethyl ethylenediamine (TMED) as the template agent, under the existence of hydrogen fluoride (with the mixture ratio of 1.0HF:1.5TMED:1Al2O3:1P2O5:80H2O, crystallized at 200° C. for 24 h). According to the above doctoral dissertation, the product with CHA framework could not be obtained as adding the divalent metal ions in the synthesis system, and when there was no fluorion in the synthesis system, the product obtained was AlPO-21. The hydrothermal synthesis of AlPO-34 and SAPO-34 with low content of silicon was reported in U.S. Pat. No. 6,835,363 using the organic amines with two dimethylamino—as the template agents, under the existence of hydrogen fluoride.
Accordance to the above report, in research of synthesis of the molecular sieves using the organic amines with two dimethylamino—as the template agents, AlPO-34 and SAPO-34 could be obtained under proper synthesis condition and existence of fluorion. When there was no fluorion in the hydrothermal synthesis system, the products obtained were the molecular sieves with other frameworks. It indicates that the fluorion plays important role in the hydrothermal synthesis of the molecular sieves with CHA framework.
It is well known that the fluorion has strong corrosiveness to steel. In the large-scale produce of AlPO-34 and SAPO-34 molecular sieves, the corrosion of steel autoclave by the fluorion in the synthesis system is a problem which cannot be ignored. It has an important scientific value and utility value to seek a fast and efficient synthesis method of SAPO-34 without use of fluorion-containing system.